Groundwater Tracer Experiments that Change the pH Show that Zinc and Phosphate can Be Released from Contaminated Sediments

Cape Cod Toxic Substances Hydrology Research Site, Massachusetts

The Cape Cod research site is near a wastewater-treatment facility at the Massachusetts Military Reservation (MMR). A treated-sewage plume originated from the facility’s infiltration beds, which were used from about 1936 to 1995. The plume extends more than 6 kilometers from the disposal site in the sand and gravel aquifer and contains a complex mixture of phosphate, nitrate, metal ions (such as zinc), detergents, organic chemicals, and microbes. The plume and adjacent parts of the aquifer serve as an ideal field laboratory where a multidisciplinary team of scientists is investigating how contaminants are transported in groundwater.

U.S. Geological Survey (USGS) scientists studying the fate of contaminants in a subsurface plume of treated sewage on Cape Cod, Massachusetts, have shown that, under some chemical conditions in the subsurface, contaminants such as zinc and phosphate can be released (desorbed) rapidly into groundwater despite decades of contact with the sediments.

What They Did

The scientists conducted several groundwater tracer experiments in which they injected into the subsurface a solution of chemical tracers and then monitored groundwater chemistry and tracer concentrations downstream of the injection well. In this case, the tracer solution was designed to create of small plume of groundwater with different chemical characteristics (more acidic pH values and higher zinc and phosphate concentrations) within the much larger treated-sewage plume. The treated-sewage plume has elevated concentrations of dissolved and adsorbed zinc and phosphate. The objective of the test was to see if the tracer plume could desorb zinc and phosphate from the contaminated sediments.

Their Results

The scientists observed that, as the tracer plume moved through the subsurface, zinc and phosphate did indeed desorb rapidly from the subsurface sediments (a sandy aquifer) in response to the chemical changes (different pH) despite having been adsorbed to the sediments for many decades. As a result of the desorption, the concentrations of zinc and phosphate in the groundwater increased. Zinc concentrations increased up to 20 times (over 2,000 micrograms per liter [µg/L]) more than the original concentration in the contaminated groundwater and to a level that is greater than the U.S. Environmental Protection Agency's recommended acute water-quality criteria for zinc (120 µg/L).

A diagram of the setup of the subsurface tracer test. The injection solution consisted of native groundwater in which the pH was decreased from an initial value of 5.9 to a constant value of 4.5. Bromide was added to the solution to act as a conservative tracer.

Computer Modeling

The scientists used computer models of contaminant transport in groundwater and reactions between the dissolved chemicals and the sediment surfaces to simulate their observations of the tracer test. The models were able to simulate the major trends in the data collected during the test. The computer models show promise as tools for predicting the fate of adsorbed contaminants in groundwater.

Applications

The results of this study and the computer model developed by the scientists can help land managers, regulators, and environmental professionals develop sound policies to protect the quality of groundwater. In addition, the models have the potential to help these professionals anticipate the effects of various activities, such as changes in contaminant source, contaminant cleanup, or land use, on the fate of contaminants adsorbed on subsurface sediments.